Joints Between Precast Concrete Elements

ABSTRACT

A joint between two adjacent faces ( 14 ) of concrete elements is formed by overlapping headed bar ( 16 ) with transverse studs ( 22 ) between the bars.

The present invention relates to the use of headed anchor reinforcementbars in the creation of joints between precast concrete elements.

Headed deformed bars are deformed reinforcing bars, such as ribbedcarbon steel reinforcing bars, with a head attached at one or both ends.Smooth reinforcing bars are also used with heads. In this specificationa reinforcing bar with a head attached at both ends is described as adouble headed stud whether or not the shank is deformed for betteranchorage.

The use of lap splices anchored by headed bars in creating jointsbetween precast elements is described in a technical paper entitled LapSplices Anchored by Headed Bars by M. Keith Thompson, Antonio Ledesma,James O. Jirsa, and John E. Breen published in ACI Structural Journal V103, No 2 March April 2006. This primarily addresses their use in bridgestructures. This paper describes the mechanics of such joints. A moredetailed report by the same authors from May 2002 has been published bythe Center for Transportation and Research, The University of Texas,Austin as Report 1855-3. These documents are herein referred to as theTexas papers. A typical joint width under consideration in thesedocuments is 10 inches (0.254 m).

Headed bars are also used in the reinforcement of flat slabs,particularly to deal with localised high shear stresses around columnheads. The RFA-Tech SHEARTECH® system proposes the use of double headedshear studs welded to carrier/spacer rails. These stud carrying railsare designed to be placed liked rays within the slab surrounding acolumn in order to provide shear reinforcement. The studs are positionedvertically within the surrounding slab parallel to the axis of thecolumn. There are other proprietary systems for use around column headsoffered by Halfen and Max Frank of Germany.

The present invention is particularly concerned with the problems of theconstruction of floors within large multi-storey structures requiringflat slab constructions to create a framework defining multiple floors.These floors can be assembled from precast concrete planks. The jointsbetween these planks and between other structural elements such asconcrete columns, beams, walls which are used in a variety ofconfigurations, must be structurally robust. It is also desirable tominimise the use of high strength concrete in making of these joints.

The present invention provides a longitudinal joint between two precastconcrete elements each having headed bars projecting from adjoiningfaces of the elements to be joined; the elements being positioned sothat their respective headed bars are interleaved and overlap but arenot in contact with one another; and headed studs positionedtransversely to a length of the joint and transversely to and betweenthe headed bars, the bars and studs being enclosed in concrete.Preferably a joint structure is employed in which headed bars areinterlaced in the manner of a lap joint as described in the Texas paperswith the addition of a stud carrying rail along the length of the jointsupported on the overlapping headed bars and suspending double headedstuds between them. When such a reinforcement structure is embedded inhigh-strength concrete, a robust structural joint of relatively smalldimensions can be constructed. In analytical terms, the transfer offorces can be considered as a series of compression struts and tensionties according to normal strut and tie theory, with the strength of thecompression struts enhanced by the confining effect of the intermediatestuds on their carrying rail.

Where joints are required to be formed above a steel beam element,single headed studs can be pre-welded to the beam top flange to providethe intersecting studs between the overlapping headed bars.

Using this form of joint construction, the precast structural elementsof the system can be manufactured as large elements. The elements willbe typically produced using lightweight concrete to reduce their weightand ease handling and transportation logistics.

Joints of the system between adjacent slab elements, beam and slabelements, wall and slab elements and other similar scenarios, requiretemporary supporting of one element from the other element. This istypically provided by temporary steel channels bolted on top of oneelement and which rest on the adjacent element through cantileveringaction.

In order that the invention may be well understood, an embodiment of ajoint between adjacent floor planks will now be described by referenceto the accompanying diagrammatic drawings, in which:

FIG. 1 shows a perspective view of an embodiment of a partly constructedfloor using the joint of the present invention;

FIG. 2 shows a perspective view of adjacent planks with reinforcement toconstruct a joint in accordance with the invention in place;

FIG. 3 shows a view of the joint from above; and

FIG. 4 shows a perspective view of an alternative embodiment of a partlyconstructed floor using the joint of the present invention.

A floor in a multi-storey construction is assembled from precastlightweight concrete (LWC) planks 10. The planks 10 are supported byconcrete beams 26 and columns 28 and prior to the construction of thejoints between the beams and planks and adjoining planks, the elementsare supported by means of steel channels 30.

Joints 12 are constructed as shown in FIGS. 2 to 3. Each edge face 14 ofa beam or plank 10 has a series of headed bars 16 projecting from it. Inthis example, the bars are shown in two vertically spaced layers. Thereinforcement is precast into the planks so that they can be laid edgeto edge with the headed bars of one plank interlaced and overlappingwith those of the adjacent plank without the bars coming into contact orconflict with one another. The edge faces of both the beams and plankshave similar arrays of headed bars at the same spacing so that joints ofthe same form can be made between all the elements.

Two longitudinal bars 18 are laid along the length of the joint and arail 20 which supports a series of double headed studs 22 rests on theuppermost headed bars. The studs are spaced along the rail so that theycan be positioned centrally between the headed bars. The double headedstuds 22 hang vertically from the rail 20.

In this embodiment, the rail comprises two rods to which the heads 24 ofthe double headed studs are welded. The rail is there to keep the doubleheaded studs 22 in position at the required spacing and performs nostructural part of the joint. It may be relatively lightweight. Thelowermost head of the double headed studs lies within the joint.

Shuttering is provided beneath the joint so that concrete can be pouredinto the gap to surround the reinforcement and bring the surface of thejoint up to the level of the adjoining surfaces of the precast planks.Once joints have been completed, the channels 30 are removed.

It is envisaged that a joint width typically of 200 mm can be employedwith a lap length of 100 mm using reinforcing bars of a typical diameterof 16 to 25 mm.

In FIG. 4, a steel framework 4 is made up of steel beams 6 that define aperimeter of the floor and at least one horizontal beam 8 within it.This framework 4 is used to support a plurality of lightweight concrete(LWC) planks 10, which are laid on the framework 4 and connected to it.The joints are then made as previously described.

Where a joint overlies a beam 6, the rail supported double headed studs,can be replaced by a series of single headed studs welded to the top ofthe beam. These perform the same function within the joint.

1. A longitudinal joint between two precast concrete elements eachhaving headed bars projecting from adjoining faces of the elements to bejoined; the elements being positioned so that their respective headedbars are interleaved and overlap but are not in contact with oneanother; and headed studs positioned transversely to a length of thejoint and transversely to and between the headed bars, the bars andstuds being enclosed in concrete.
 2. A joint as claimed in claim 1,wherein the headed studs are double headed studs supported on a railwhich rests on the overlapped headed bars.
 3. A joint as claimed inclaim 1, wherein the headed studs are welded to a beam beneath thejoint.
 4. A joint as claimed in claim 1 wherein each element edge has adouble row of headed bars.
 5. A joint as claimed in claim 1, wherein atleast one longitudinal bar is positioned along the length of the joint.6. A joint between two precast concrete elements substantially as hereindescribed with reference to the accompanying drawings.